1. Field of the Invention
The present invention pertains to, but is not limited to, a system for recirculating the gas atmosphere within an excimer laser system, where contaminates, created in the laser's operation, are removed, and the gas concentrations of additive gases, such as krypton (Kr), xenon (Xe), or others, depleted in the laser operation, are rebalanced to specified lasing mixture concentrations by analyzation and component replenishment from one or more external supplies. This system prevents the loss of significant amounts of the laser gas mixtures, which is important since gases such as Neon (Ne), which can account for approximately 97 percent of the laser gas mixture, are and expensive due to shortages, and lost once vented.
2. Description of the State of the Art
Excimer lasers are pulsed gas discharge lasers which produce optical output in the ultraviolet region of the spectrum. There are four commonly used excimer wavelengths which are dependent upon the active gas fill of the laser, the four wavelengths are:
WavelengthActive GasRelative Power193 nmArgon Fluoride (ArF)60248 nmKrypton Fluoride (KrF)100308 nmXenon Chloride (XeCl)50351 nmXenon Fluoride (XeF)45
An excimer lasers are now commonly used in the production of microelectronic devices (semiconductor integrated circuits or “chips”), eye surgery, and micromachining. To operate efficiently, excimer lasers require three or more part, mixtures of rare high-purity noble gases such as krypton (Kr), xenon (Xe), or argon (Ar), and consequently the operation of an excimer laser is expensive. In addition to using rare high-purity noble gases a highly reactive halogen gas, such as, fluorine (F), or chlorine (Cl), apart from helium (He) and/or neon (Ne) as buffer gas, is further utilized. Since such small amounts of Xe are used, and there is a tremendous supply of Ar, there is no concern for recovery for these gases. Furthermore, the buffer gas is the primary gas in the lasing mixture, accounting for up to 99% by weight. The buffer gas also has to be chemically resistant in the excimer lasing gas chamber, as the halogenated gases, such as F2 and Cl2, will react with just about any elements and/or molecules when atomized in the excimer lasing gas chamber. The choices for the buffer gas are He or Ne, and since He has a limited supply and is not recoverable once released to the atmosphere, Ne is the predominant buffer gas used in excimer lasers. With the growing amount of excimer lasers use in the world, there is a concern for Ne shortages, therefore the price of this noble gas has increased dramatically, and the growing need for lasing gas recovery. These gaseous components, and possibly other gases, are contained within a pressure vessel provided with longitudinally extending lasing electrodes for inducing a transverse electrical discharge in the gases. The discharge causes the formation of excited rare gas-halide molecules whose disassociation results in the emission of ultraviolet photons constituting the laser light. During operation, the halogen gas component reacts with materials inside the laser, such as, C, H, and is depleted from the gas mixture requiring periodic replacement. Halogen depletion coincides with the formation of impurities within the laser chamber, which impairs laser operation reducing the laser output power.
In order to maintain a constant power from the laser, the voltage applied to the laser's electrodes can be increased to overcome the reduction in output power caused by the contaminants and the depleted halogen. Unfortunately, the higher voltages lead to a more rapid deterioration of the electrode materials in the laser, and a large increase in maintenance costs. A portion of the laser output energy can be recovered by simply replacing the depleted halogen in the laser chamber; however, without a means to remove the impurities, the laser gas mixture must be eventually replaced to return the laser back to full output.
Accordingly, a significant portion of the operating cost of an excimer laser is therefore related to the contamination of costly, high-purity, noble gases. Over the years many of the challenges associated with excimer lasers have been mitigated through the use of corrosion-resistant materials, advanced gas recirculating and purification systems, and solid-state high-voltage switches. These continued engineering improvements and rise of applications continue to exert a high demand on rare high-purity noble gases. For example, it has recently been demonstrated that a very narrow band pulse excimer laser capable of producing pulses at a rate in the range of about 500 to 2000 Hz with enhanced energy dose control and reproducibility can be achieved by adding small quantities of a laser enhancer consisting of oxygen or a heavy noble gas (xenon or radon for KrF lasers, or krypton, xenon or radon for ArF lasers) to the gas mixture. Tests demonstrated improved performance for the ArF lasers with the addition of about 6-10 ppm of Xe or 40 ppm of Kr.
Accordingly, an improved system for reclaiming, rebalancing and recirculating rare high-purity noble gases and specifically xenon, is needed to ensure a continued supply of these gases at acceptable prices. An object of the present invention is to overcome the shortcomings of the prior art by providing an apparatus and method, which incorporates a unique change to the design concept of an excimer laser, whereby the expensive noble gases are reclaimed, rebalanced and recirculated while removing the impurities developed during operation of the excimer laser.